This invention relates to a positive electrode for an alkaline storage battery comprising nickel oxide as the main component and an alkaline storage battery using the positive electrode, particularly to a nickel-hydrogen storage battery having improved characteristics.
Recently, there has been an intensive need for high energy density secondary batteries associated with portable appliances which are made much more valuable, compact and lighter in weight. Similarly, such new high energy density secondary batteries are also vigorously sought for power supplies for electric automobiles. To meet to these desires, in the technical field of nickel-cadmium batteries (referred to hereinafter as the Ni-Cd battery), there has been developed a Ni-Cd battery having a higher capacity using a conventional sintered nickel positive electrode, and further a much higher energy density Ni-Cd battery using a foamed metal type nickel positive electrode having a capacity of 30 to 60% higher than that of the sintered nickel positive electrode. Moreover, there has been developed a nickel-hydrogen storage battery having a higher capacity than those of Ni-Cd batteries (the higher capacity is at least two times that of the Ni-Cd battery using the sintered nickel positive electrode) by use of a hydrogen storage alloy as a negative electrode. In these high capacity alkaline storage batteries, a three-dimensional porous body such as a bulk formed porous nickel body or a porous fibrous nickel body having a high porosity (at least 90%) is filled with a nickel hydroxide powder at a high density in order to achieve an improvement in capacity of the positive electrode. As a result, the capacity density has been increased thereby to 550 to 650 mAh/cm3 while the capacity density of the conventional sintered nickel positive electrode is 400 to 500 mAh/cm.sup.3.
These nickel positive electrodes, however, have a common problem that their energy density can be maintained at ordinary temperatures, but reduced at high temperatures atmosphere. Accordingly, it has been difficult to take advantage of the characteristics of high energy density in a broader temperature range. The cause thereof is that, in the charging under a high temperature atmosphere, an oxygen evolving reaction tends to be caused simultaneously with the charge reaction in which nickel hydroxide is charged into nickel oxyhydroxide. That is, the oxygen evolving overvoltage at the positive electrode is reduced, whereby nickel hydroxide is not sufficiently charged into nickel oxyhydroxide and the charging efficiency of the positive electrode is reduced, so that the utilization of nickel hydroxide is lowered. To solve this problem, the following methods have been proposed:
(1) A method in which a cadmium oxide powder and a cadmium hydroxide powder is added to the positive electrode. PA1 (2) A method in which cadmium oxide is incorporated into a nickel hydroxide powder (see JP-A-61-104,565). PA1 (3) A method in which a compound comprising yttrium, indium, antimony, barium or beryllium is incorporated into the positive electrode (see Japanese Patent Application No. 4-248 973 publication No. P-A-6-103,973. PA1 (4) A method in which a compound of erbium or ytterbium is added alone to the positive electrode [see Reprint of the 63rd Electrochemistry Society Spring Convention (1996)].
In the above-mentioned methods (1) and (2), the presence of cadmium oxide added or incorporated into the nickel hydroxide powder improves the rate of utilization of the nickel hydroxide under a high temperature atmosphere. However, even in these cases, the rate of utilization of nickel hydroxide under a high temperature atmosphere is usually about 80%. In order to further increase the rate of the utilization, it is necessary to increase the amount of cadmium oxide to be added, and by increasing the amount of cadmium oxide, the rate of the utilization of nickel hydroxide under a high temperature atmosphere can be increased to about 90%. However, the increase of the amount of cadmium oxide to be added causes a problem of adversely diminishing the rate of the utilization of nickel hydroxide at ordinary temperatures. Moreover, from the viewpoint of environmental pollution, a nickel-hydrogen storage battery which does not contain cadmium is preferred. However, when cadmium oxide is not added, the utilization of nickel hydroxide is lowered to about 50 to 60%.
The above-mentioned method (3) has been proposed to solve these problems. According to this method, the oxygen evolving overvoltage at the time of charging under a high temperature atmosphere is increased by absorbing a compound of yttrium or the like on the surface of nickel oxide, and the charge reaction of nickel hydroxide into nickel oxyhydroxide is sufficiently effected, whereby the utilization under a high temperature atmosphere is increased. According to this method, it is possible to increase the utilization of nickel hydroxide at 45.degree. C. to about 80% or more. Also, it has been reported that in the above method (4), the same effect is obtained by adding a compound of erbium or ytterbium alone.
However, in order to meet a recent desire of making the capacity higher, it is necessary to develop an additive which, even when added in a small amount, acts effectively to enhance the charge efficiency under a high temperature atmosphere, thereby further increasing the utilization of nickel hydroxide under a high temperature atmosphere. Moreover, it is considered necessary that the utilization be further increased by a method which comprises adsorbing said effective additive on not only the surface of the nickel oxide but also the surface of the support and the surfaces of positive electrode-constituting materials such as cobalt, cobalt hydroxide, cobalt oxide, zinc oxide, zinc hydroxide and the like to enhance the oxygen overvoltage of the whole of the positive electrode plate. In addition, it is considered necessary to uniformize the reaction in the positive electrode plate to prevent the swelling of the active material. From such a viewpoint, the problem to be solved by this invention is to enhance the utilization of the active material under a high temperature atmosphere and further enhance the cycle life.